Nuclear power plants of the type in which a nuclear reactor generates heat which is ultimately converted into electrical energy, generally operate with a plurality of coolant-circulating systems, e.g. a primary coolant system and a secondary coolant system, which ultimately transfer the heat to a fluid such as steam, which, in turn, can be used to drive turbines for the generation of electrical energy.
In many cases, the primary coolant system is operated with water to which boric acid is added, the primary coolant system circulating the cooling directly through the core of the nuclear reactor.
From such nuclear power plants a waste-water concentrate is obtained which may contain, apart from the boric acid, various radionuclides and chemical residues.
Such waste water may be recovered from evaporator units utilized in the processing of the waste-water, from leakage from the primary system in which the coolant contains boric acid or as sump waters. The concentrate usually contains boric acid up the solubility limit and can contain, as radionuclides, Cr-50, Mn-54, Co-58, Co-60, Sb-124, Cs-134 and Cs-137. The concentrate also can contain corrosion products, suspended matter, surface active agents and complex-forming chemicals as the chemical residues.
The boric acid concentration in the practically neutral waste-water concentrate can be about 40 g/l or above.
In most cases, such waste-water concentrates are generally packaged or stored as is the case with other radioactive wastes in an extremely costly operation.
To reduce the amount of radioactive waste which must be stored, a method has been proposed (see German open application - Offenlegungsschrift - DE-OS No. 27 23 025) whereby boric acid is recovered and can be recycled to the primary coolant system of the reactor.
In this process, the waste-water concentrate is initially subjected to concentration by boiling and the boric acid in the concentrate is transformed, with methanol, into the boric acid trimethylester. The latter is distilled off and converted again to boric acid and methanol by hydrolysis.
The boric acid has a sufficiently high purity that it can be directly utilized in the primary system.
This process involves various problems because of the use of methanol which is highly volatile and combustible.
As a result, the process involves hazards to operating personnel and may not satisfy environmental protection standards. Difficulties are encountered in the use of organic solvents which result in iodine residues in the filters.